Stable isotope records (δ18O and δ13C) of Lower-Middle Jurassic belemnites from the Western Balkan mountains (Bulgaria): Palaeoenvironmental application

Stable O and C isotope data of 110 Upper Pliensbachian-Lower Bajocian belemnites have been obtained and used to attempt a reconstruction of palaeotemperature and its variation in two epicontinental depositional environments from the Western Balkan mountains (Bulgaria). The samples were collected from 3 sections with high-resolution ammonite subdivision. Initially taphonomic, cathodoluminescence and geochemical analyses were used for evidence of diagenetic alteration. Non-luminescent parts of the belemnite rostra have been sampled for isotope analyses and 76 samples, having δ18O < −4‰ (PDB), δ13C > −0.5‰ (PDB), Fe < 250 ppm, Mn < 50 ppm, Sr > 950 ppm and Sr/Mn ratio > 80 were used for palaeotemperature interpretations. The O and C isotope data generally exhibit little stratigraphical variability with minor fluctuations, however, there are prominent positive C isotope excursions and coeval negative O isotope shifts detected in the Lower Toarcian Tenuicostatum, Falciferum and Bifrons Zones. The O isotope data, interpreted in terms of palaeotemperature, revealed relatively high seawater temperatures during the Toarcian, Aalenian and Early Bajocian, with detectable temperature rises during the Early Toarcian (maximum value of 29.6 °C). Both C isotope maxima and O isotope minima are used as evidence of the Early Toarcian anoxic event reported from many localities of the same age and in similar facies in Western Europe. In the study the latter is recognized as 3 episodes, which are closely related with the seawater temperature maxima. This isotope record pattern is considered as a consequence of a global Tethyan transgression during the Early Toarcian.